1. Field of the Invention
A present invention relates to an automatic control method for growing single-crystal neck portions by the CZ method.
2. Description of the Related Art
In single crystal growing devices employing the CZ method, an automatic control method has been developed for a process of growing the cone portion of a crystal, but manual control by a skilled worker is still required for a process of growing the neck portion, i.e. the process between the initiation of crystal growth and the growing of the cone portion. Because dislocations in the neck portion must be removed through the crystal surface, highly sophisticated control is required, for example, restricting the diameter of the growing crystal to a range of about 2 to 5 mm, pulling up the crystal at a relatively high speed, maintaining the absolute value of the diameter control deviation at about 0.5 mm or less, and growing the neck portion to a height more than ten times the diameter thereof. It is difficult even for an experienced worker to thoroughly remove dislocations from the crystal and to obtain a desired shape of the crystal during the processes of thinning and then thickening the crystal diameter. Usually, about 10% of the operations attempted by such workers result in failure. If the diameter of a growing crystal is thinned to be too small, a breakage may occur between the growing crystal and a melt at the interface therebetween to make it impossible to continue the growing thereof, or the growing portion may fail to acquire a strength sufficient to support a body portion which is grown afterwards. If the crystal diameter is too large, insufficient removal of dislocations may result, causing the existence of dislocations in the cone and body portions of the crystal and thus causing defected crystals to be produced.